In olefin epoxidation, an olefin is reacted with oxygen to form an olefin epoxide using a catalyst comprising a silver component, usually with one or more elements deposited on a carrier. Catalyst performance is characterized on the basis of selectivity, activity and stability. Moreover, the performance in the reactor tubes is characterized by the packing density of the catalyst in the volume of the tubes and pressure drop.
The selectivity is the molar fraction of the converted olefin yielding the desired olefin oxide. Quite modest improvements in selectivity and the maintenance of selectivity over longer time yield huge dividends in terms of process efficiency.
Packing of the catalyst in the reactor tube depends on the geometrical size and shape of the carrier that the catalyst is deposited on. Typically higher packing density, i.e., more catalyst in the volume, is considered as advantageous.
Alumina is well known to be useful as a carrier for olefin epoxidation catalysts. Carrier materials including alumina are typically made by fusing high purity aluminum oxide with or without silica. For this purpose, the carrier material often comprises 90 percent or more by weight alpha alumina, and 1 to 6 percent by weight silica. The carrier may be very porous or non-porous and have a high or low surface area depending upon the intended use of the carrier. The carrier may contain any porous material that does not detrimentally influence the catalytic reaction where it is used.
In the process of making a carrier, high-purity aluminum oxide, preferably alpha alumina, is thoroughly mixed with temporary and permanent binders. The temporary binders are thermally decomposable organic compounds of moderate to high molecular weight which, on decomposition, produce the desired pore structure of the carrier. The permanent binders are inorganic clay-type materials having fusion temperatures below that of the alumina and impart mechanical strength to the finished carrier. After thorough dry-mixing, sufficient water or another solvent is added to the mass to form the mass into a paste-like substance. The carrier particles for making the catalyst are formed from the paste by conventional means such as, for example, high pressure extrusion, granulation or other ceramic forming processes. The particles are then dried and are subsequently fired at an elevated temperature.
In the firing step, the temporary binders are thermally decomposed and are volatilized, leaving voids in the carrier mass. These voids are the genesis of the pore structure of the finished carrier. As firing continues, the temperature reaches the point at which the permanent binder turns into inorganic clay. The catalyst carrier is then cooled, and during cooling the permanent binder sets, acting to bond the carrier particles, and thereby impart mechanical strength to the carrier and ensure maintenance of the pore structure.
Catalyst carriers of desired characteristics can be readily produced by the foregoing procedure. Pore size, pore distribution and porosity are readily controlled by appropriate adjustment of the size of the starting alumina particles, and of the particle size and concentration of the temporary and/or the permanent binders in the mixture. The larger the starting alumina particle size, the greater the porosity of the finished catalyst. The more homogenous in size are the alumina particles, the more uniform the pore structure. Similarly, increasing the concentration of the temporary binder also increases the overall porosity of the finished catalyst carrier.
The preparation of alumina carriers of particular physical properties and porous structure for ethylene epoxidation catalyst performance enhancement are described, for example, in U.S. Pat. Nos. 4,226,782, 4,242,235, 5,266,548, 5,380,697, 5,597,773, 5,831,037 and 6,831,037 as well as in U.S. Patent Application Publication Nos. 2004/0110973 A1 and 2005/0096219 A1.
In addition to the preparation of alumina carriers, the prior art also describes the preparation of alumina shaped carriers for catalytic applications in various shapes for different purposes. For example, U.S. Pat. No. 2,950,169 describes a method of preparing pills of high crushing strength. For many uses, it is preferred that the alumina be in particles of uniform size and shape, including, for example, cylindrical pills, spheres, polyhedra, tubular cylindrical pills, etc.
U.S. Pat. No. 3,222,129 relates to an active alumina product which has improved resistance to attrition, abrasion and crushing. Alumina may be manufactured in shaped nodules or pellets, etc., in the forms of spheres, or the like, preferably of uniform size and shape. The shaped nodules are particularly desirable because they can be more easily handled than granular or particulate material.
Alumina carriers for ethylene epoxidation in various shapes, such as balls, granules, rings or the like, as described, for example, in U.S. Pat. No. 3,172,866 are known, however, little is known about the shape and performance relation. Among such examples are U.S. Patent Application Publication Nos. 2004/0260103 A1 and U.S. 2004/0225138 A1 which describe a catalyst and reactor system for the manufacture of ethylene oxide. This prior art catalyst was made on a carrier of a particular geometrical configuration. A proper selection of geometrical size of a hollow cylinder reportedly increased the packing density at acceptable pressure drop values in the reactor. It was specifically identified that an advantageous support material has a hollow cylinder geometric configuration defined by a ratio of nominal outside diameter-to-nominal inside diameter. The smaller than conventional bore diameter helps provides for an improvement in the average crush strength of the agglomerate, and provides for packing a greater amount of support material into same volume, allowing for more silver to be loaded into the same volume.
As described above, a catalyst for ethylene epoxidation requires a carrier with specific physical properties and those carriers can be shaped in various shapes of different sizes. The carrier properties and shape are important for loading catalytically active silver in the reactor tubes. It would be desirable to improve performance and effectiveness of a catalyst used in the reactor tubes by improvements in physical properties and geometrical shape of the carrier.